The growth of single crystals of various ceramic materials according to a process known as the "gradient furnace" technique is disclosed in U.S. Pat. No. 3,653,432 issued to Schmid and Viechnicki. In accordance with that process, the uni-directional solidification of high temperature ceramic materials to produce single crystals is accomplished by seeding the melt of a ceramic material, melting the ceramic material under vacuum conditions while uni-directionally cooling the melt by passage of a coolant gas against the closed end of a heat exchanger located at the bottom portion of a crucible. The material is caused to uni-directionally solidify by decreasing the temperature of the melt while increasing the flow of coolant gas. The size of crystals grown according to this technique, however, is limited by the relatively small size of the crucibles which must be utilized in order to achieve complete melting of the material.
It is generally known that single crystals of yttrium aluminum garnet (YAG) provide an excellent laser material when approximately one atomic percent of Nd.sup.+3 ions are dissolved and retained therein during crystal growth. However, it has been found that the Nd.sup.+3 ions are not evenly distributed during crystal growth because the Nd.sub.+3 ions are rejected at the growth interface and the molten ceramic material tends to build up a concentration of Nd.sup.+3 ions which result in an undesirable range of Nd.sup.+3 concentration throughout the single crystal ingot. This has been found to be the case regardless of whether the crystal is grown according to the gradient furnace technique or any of the other known techniques for growing YAG crystals.
Single crystals of materials such as germanium, which are used in the semiconductor industry, have been grown according to a technique known as "zone leveling". This technique, which is particularly useful when it is desired to introduce impurities into the crystal, is described in great detail in a book by William G. Pfann entitled "Zone Melting" published by John Wiley & Sons, Inc., New York, 1958. According to this technique, a floating molten zone is used to melt successive portions of the material from which the crystal is to be grown. However, it has not heretofore been known that the technique of zone leveling could be combined with the gradient furnace technique to grow single crystals of ceramic material which would be virtually limitless in size, and could be grown with a constant concentration of dopant throughout the crystal where such constant concentration result is required such as, for example, in the case of Nd.sup.+3 doped YAG for laser applications.